This invention pertains generally to active radar seekers for guided missiles and particularly to a method for encoding a high pulse repetition frequency (PRF) pulse train waveform to produce range and Doppler resolution characteristics approximating those of a low PRF waveform.
Recent and ongoing advances in the design of solid state transmitters such as those described in the pending U.S. patent application entitled "Solid State Power Combiner", Ser. No. 814,746, filed Jun. 30, 1977, Inventors Jerinic et al, (which application is assigned to the same assignee as this application) have resulted in a class of compact, lightweight, solid state transmitters particularly well suited for use in active radar seekers for guided missiles. In such an application operation is usually carried out with a high duty factor (ideally around 30 percent) in order to provide high average power for optimum target detectability and increased target detection range. A 30 percent duty factor presents no problems with high PRF waveforms. However, with low PRF waveforms a 30 percent duty, factor would result in relatively long pulse widths which are not easily realized with known solid state transmitters because of the thermal and RF impedance matching problems encountered with the oscillators (IMPATT diodes or the like) in such transmitters.
It will be appreciated by those of skill in the art that pulse train waveforms are described as being of high, medium or low PRF to indicate the relationships which exist between waveform ambiguity spacings and the extent of the clutter-target complex in range and frequency. Thus, for example, the PRF is considered to be high when the spread of any clutter and all targets of interest is less than such frequency and the PRF is considered to be low when the range ambiguity is greater than the maximum target range. That is to say, a high PRF waveform is one which eliminates Doppler ambiguities and a low PRF waveform is one which eliminates range ambiguities. It follows then that a medium PRF is one which allows range and Doppler ambiguities within the zone occupied by clutter and targets. Each PRF possesses peculiar advantages and disadvantages.
As is known, the full advantage of the high PRF waveform is realized in the case of approaching targets. In such case, targets exhibit positive Doppler shifts which may be great enough to move the target echo signals to frequencies which are substantially higher than those of any clutter signals. There are many known ways, then, to effect high pass filtering to separate target echo signals from clutter signals and to process only the target echo signals to derive the requisite guidance commands. Crossing targets, on the other hand, exhibit little, if any, Doppler shifts so the target echo signals remain in the same frequency region as any clutter signals. Further, in the case of the crossing targets illuminated by a high PRF waveform range ambiguities exist which lie along lines which are almost parallel to the strongest main lobe clutter contour. Because the target echo signals from the crossing targets must compete with the sum of the clutter signals the high PRF waveform is the worst possible choice for distinguishing crossing targets in a high clutter environment, as, for example, in a so-called "look down" engagement.
The effect of a high PRF waveform in the case of a receding target is somewhat more complex. Thus, when competing clutter signals are of little import (as when a guided missile and a target are at high altitudes so that the amplitudes of ground clutter signals are low and there is no clutter from precipitation) operation is satisfactory. In situations when competing clutter signals have relatively high amplitudes it becomes very difficult to distinguish between target echo signals from receding targets and clutter signals. Thus, when the altitude of either the guided missile or a desired receding target is low, i.e. in either a so-called "look up" or "look down" situation, the power of ground clutter signals (received in the "look up" situation through sidelobes and in the "look down" situation through the main lobe) usually is high enough to mask the target echo signals. Even when ground clutter signals are not large enough to prevent target echo signals from being distinguished, clutter signals from precipitation, as rain, may be encountered. In such a situation (which is analogous to the "look down" situation in teat main lobe clutter is experienced) the amplitude of the clutter signals may also be so high that target echo signals cannot be distinguished.
It will now be apparent that, while a high PRF waveform is the optimum waveform for use against an approaching target, a waveform which is better fitted to other tactical situations would be desirable. The most obvious change in the PRF waveform would be to employ a multiple, or staggered, PRF waveform so selected that the blind region of either mainlobe or sidelobe clutter signals is within a clear region. However, as noted hereinabove, multiple PRF waveforms are not easily realized with a solid state transmitter using IMPATT diodes operating at a 30 percent duty factor. Therefore, other waveforms, incorporating binary and polyphase coding of the carrier phase as well as pulse-to-pulse frequency coding, are better fitted to resolve the range-Doppler ambiguity problem when IMPATT diodes are involved.
In one known coding approach a pseudo-random coded pulse waveform which consists of some number, N, of sequential rectangular pulses, or, equivalently, a single pulse divided with N contiguous subpulses, may be used. The R.F. phase of each pulse, or subpulse, in the waveform is set to either 0 or 180 degrees in accordance with a randomly or systematically derived sequence. The particular sequence chosen is often one which creates uniformly low sidelobes in the ambiguity function along the range axis (i.e., the autocorrelation function of the waveform). However, such sidelobes cannot be maintained over the entire range-Doppler space so the use of the pseudo-random coded pulse waveform is limited in practice to situations where the Doppler shifts of target echo signals are confined to a narrow band of frequencies.
In another coding approach, a maximal-length sequence binary code is used to impose a phase shift of either 0.degree. or 180.degree. on a pulse train waveform in accordance with a stored algorithm. A code of this type yields an ambiguity function characterized by major peaks of amplitude "N" (where "N" is the number of pulses in one code cycle) spaced in time by "NT" (where T is the interpulse period of the transmitted code) along the range axis. Between the major peaks, however, are minor peaks of unit amplitude and spacing "T". These minor peaks have significant impact on performance when applied to an active seeker waveform for use in a high clutter environment. Further, and of even greater importance, ridges having an rms amplitude of N appear at harmonics of the code repetition frequency.
In another coding approach a so-called "Frank code", described in "Radar Signals" by C. E. Cook and M. Bernfield, Academic Press, New York, 1967, Chapter 8, pg. 225, is used. With such a code no minor peaks appear between the major peaks on the range axis, but, at harmonics of the code repetition frequency, noise-like peaks of amplitude N and spacing "T" do occur. As a result, therefore, nearly the same performance limitations are applicable as when a maximal-length binary code is used.
Other pulse coding approaches employ pulse-to-pulse changes in frequency rather than phase. Two variations of such coding approaches which produce nearly the same result have been implemented. In one known implementation a number of frequency sources, each locked to the harmonics of a common difference frequency, are sequentially selected to produce a waveform which may be considered to be a periodic staircase of frequency versus time. In the second known implementation a repetitive frequency ramp is sampled periodically by a train of pulses. The ambiguity function for either coded waveform exhibits minor peaks, between the major peaks, along the lines of constant Doppler with delay spacing T, and an amplitude which is a function of pulse width. Such minor peaks impose a performance limit on the waveform which becomes increasingly more difficult to meet with increasing duty factor.